1. How to add or avoid blank space top or bottom for a widget?
Go to Appearance > Appearance > Widgets Now collapse your widgets and you can add Top and Bottom margin for widgets. So you can recover the blank space using this margin options.
2. How to avoid or add empty space between blocks?
We have a option to add empty space between the blocks. And it can help us to make a good looking web layout. Because margin & padding is most important for design. And we can manage this empty space with this empty space element of visual composer.